Much confusion exists about Lithium batteries in model flying circles, and this article is an attempt to explain some of the do’s and don’ts and what some of the terminology means, to make it simpler for those new to flying, new to electric flying, or new to LiPo batteries. At the same time, it includes information that may be of benefit to the experienced LiPo user.

One of the things that confused me most to start with was terms like 3S1P. This refers to the pack configuration, and simply means the number of cells in Series and in Parallel. Connecting cells in Series (+ to -, like in a torch) increases the voltage, so with a single LiPo cell having a nominal (It’s voltage at half charged) voltage of 3.7v, two in series is 7.4v and 3 is 11.1v. (A single NiCad/NiMh cell is 1.2v, so a 3cell LiPo is the equivalent of a 9 or 10 cell NiCad). Connecting in series does not increase the amperage of the battery pack, but connecting them in parallel (+ to +, - to -) does. So if you connect 3 1000mah cells in series (3S1P) you will have an 11.1v 1000mah pack. If you add another similar pack in parallel (3S2P) you will have a 6-cell pack, still with 11.1v, but now with 2000mah.

The effect of this on a model plane is that it will fly for twice the time. (It will of course weigh a little more.)

Another confusing aspect was the C rating on the batteries. All good LiPo’s have a phrase like “12C continuous, 20C burst” on the label. This refers to the rate at which it is safe to pull power from the battery, something that we did not have to worry too much about with NiCad’s. They would just get hot! But Lipo’s can explode in a chemical fire that cannot be extinguished! They can also be damaged to the point that they are useless. But it is fairly simple to work out. Multiply the C figure by the amps in your pack.( 1000ma =1amp) If our two battery packs above are marked “12C continuous, 20C burst”, this translates to (12 x 1000mah [1 amp] ) = 12 amps continuous or (20 x 1000mah ) 20 amps for a few seconds. Our second pack, the 3S2P, you have doubled the mah capacity of the pack, so you can now pull 24 amps continuously and briefly 40amps.

Some older packs or cheap imported ones do not have a C rating marked on them. To determine the safe amperage for this battery, a special test meter is used that plugs in between the motor and battery. This sort of meter which is sold by many manufactures is probably one of the best electric flight items you will ever buy and it is highly recommended that you purchase one. The one pictured above is from Aurorra Ltd (www.aurorra.co.uk) where Mike offers an excellent service.

A LiPo battery will not recover if it goes below 2.5 volts per cell in series. Hook the battery and meter to a motor sufficiently powerful to over-use it. Then gradually increase the throttle until the meter shows approximately 3.2v per cell (i.e. 6.4v or 9.6v for 2 and 3 cell packs respectively) Look at the amperage at this point, and this is the maximum you can pull with this battery continuously. You can probably go 40% higher briefly, but only for 5 seconds or so. Since there is an inevitable loss of power over time, with both motor and battery wearing, 0.7v is a reasonable safety margin.

When considering whether to use LiPo batteries in any given model, or which battery will suit, it is first necessary to determine what amperage your motor and prop will pull. The only way to do this for yourself is by means of the meter used above. Most modern brushless motors will have maximum amperage it is safe to use them up to, and many manufacturers publish the current draw of their motors with props of a various sizes, either on their website or in the instructions with the motor. This is a satisfactory rough guide, but as individual components vary, and a prop by one manufacturer may give a different reading than one by another maker, it is always wise to check! And remember that a freshly charged battery will push a lot more current than one that has had a few seconds run. Just when you need full power to take off with a newly charged battery is when you are most likely to over-stress your batteries.

LiPo’s should only be used with ESC’s that have a means of cutting the power to the motor when the battery reaches a predetermined voltage. Some ESC’s are programmed via your transmitter and some have a cable which you can connect to a computer. You have to make sure you have set this up correctly for the number of cells you are using. This is very important. The other fact to bear in mind is that most of the ESC’s programmed via your transmitter will only be set up on a cell count and the manufacturers will probably have set the voltage of this to 2.8v per cell. This will prevent the cell from being destroyed but probably won’t help the long term life of the cell. When you are flying you will notice a drop in performance and this is the time to land. This will keep your cells in good condition. With the P/C programmed ones you can usually set the cut off voltage yourself with a soft ramp down which will give you time to land on reduced power (the current is reduced so the voltage goes up).You must not over discharge your LiPo. Anything under about 2.5 volts per cell is critical and the LiPo is probably destroyed and could cause a fault on charging.

Charging LiPo batteries is another area of confusion. Because they need special consideration when being charged, it is ABSOLUTELY ESSENTIAL to use a lithium compatible charger, and to TAKE CARE when setting it. Although faster charging batteries are being developed, currently it is necessary to charge at 1 C. So for our first pack, set the charger for a 1amp charge. Because the charger uses this as a maximum figure, charging a LiPo normally takes 11/2 hours or more. Another difference with LiPo’s is that balancing the cells is advisable. Often, all the cells in a battery will have a minute difference in voltage, and many manufacturers fit a small plug to attach a balancing unit.

This reduces the voltage to the same level in all the cells. But unfortunately as yet there is no standard type of balancing socket, so all the different manufacturers sell their own balancers. You can use some of the new multi cell balancers which will do two to four cells or perhaps two to eight cells and make up leads to fit your packs. Most manufactures sell leads for their cell, which you can connect to your balancer plug. Make sure the balancer is suitable for the cells you are using. DO NOT DO THIS UNLESS YOU ARE SURE OF WHAT YOU ARE DOING-GET SOMEONE WHO DOES OR ASK THE MANUFACTURE IF THEY DO THE LEAD.

If you have connected two packs in parallel to increase their capacity make sure they are in exactly the same state of charge and measure their voltage (with the new meter you have just bought) before connecting them. This is most important if you are going to charge them in this configuration. The safest thing to do is to separate them and charge them as two separate batteries. This will then overcome the problem of the two packs not being balanced to each other exactly.

It is very important that the connections to the cells cannot short out under any circumstances so only use a female deans type connector or gold connectors with a female on the + side and a male on the _ side and shorten one lead so the ends cannot come into contact . Use good quality heat shrink to protect them. You can also slide a piece of tube over the male plug when not being used.

Storage of LiPo batteries is relatively easy. Unlike Nickel batteries, they lose a very small amount of power and so can be charged and stored ready for use. But research has proved that they last best if stored in a cool place at a 40% charge. Since very few of us have the means to test the charge percentage, 3.5v per cell would be close, measured on a voltmeter

Safety TipsCharging

• Be absolutely sure that the Lithium Polymer charger settings are correct for the battery pack being charged – both voltage and current settings
• Care should be taken to charge on a fireproof surface, such as brick.
• Do not charge batteries near flammable items or liquids
• Keep a dry fire extinguisher nearby - or a large bucket of dry sand, which is a cheap and effective extinguisher.
• Do not charge inside an automobile, especially while driving
• Batteries should NEVER be left unattended while charging

The charge voltage of a lithium cell goes down with temperature. At room temperature the charge voltage is 4.2V, @ near freezing it is about 4.1V. If you charge at room temperature fully and then take the pack on a winter’s day to the flying field (or store it) and its temperature gets down to 10degC or less it will may "puff" because it will be
overcharged at this temperature! Better to charge to 80% (4.1v/cell) or run the battery down a bit on a cold day or putting in storage.

Handling Cautions:
New cells may have a high initial charge, and care must always be taken to insure that loose cells are not short circuited. Please:
• Do not put the loose cells in a pocket, bag, or drawer where they could short-circuit against other items, or the battery tabs could be pressed against each other.
• Do not place the loose cells on any conductive surface, such as a metal-topped table.
• Keep loose cells and battery packs WELL out of reach of children!
• Buy pre-assembled packs from an authorized dealer, rather than assembling packs from loose cells. You need specialist solders to connect onto the cell ends anyway
• Take care that the cells are not punctured, especially by metallic objects like hobby knives.
• If the electrolyte in the cells should get on your skin, thoroughly wash with soap and water. If in the eyes, rinse thoroughly with cool water. Immediately seek medical attention for this, or for burns.
• Although environmentally friendly, lithium polymer cells must be FULLY discharged before disposal. Use a resistor setup (light bulbs, for example) to accomplish this, to avoid the possibility of a short-induced fire after disposal.
• A punctured pack may be left OUTDOORS in salt water for 2 days to discharge.

Conclusion

Lithium Polymer or Lithium Ion cells offer many advantages for the model flier. Their small size and light weight makes them ideal for use in electric models, and they are an ideal companion for brushless motors. Because they are small, they will fit in models that would previously take a low voltage and small amperage NiCad. The lighter weight means lower wing loading on the wings, improving low speed handling, the aerobatic performance and higher top speeds. So a training model will be easier to take off and land, and will fly better and a performance model will be better in all respects, with easier manoeuvres. The model will also have less kinetic energy to dissipate in the event of a mishap, so it is likely to be less severely damaged.

However, these batteries do present some dangers, and care must be taken with them, more care than NiCads. Some investment is required to buy a suitable charger, a wattmeter and balancing unit, and the cost of the battery itself is normally higher, although the costs are coming down as the technology improves.

The dangers are minimal if suitable precautions are taken as outlined above, and the benefits are considerable. I think that in a few years time Nicads will have gone the way of tissue and dope, and we will all be using Lithium or some other future development. But for the next few years, LiPo power is king for electric models.

Acknowledgements.
My thanks to Simon Willey for his invaluable technical guidance, checking my facts and writing sections for inclusion, as well as suggesting a source of further information, Mike Redmond of Aurorra Ltd (www.aurorra.co.uk) who provided some excellent information, as well as a lot of support.